1. Field of the Invention
This invention relates to a rubber composition and a pneumatic tire comprising the rubber composition as a tread rubber and/or a sidewall rubber having improved tear resistance, cut resistance, wear resistance, resistance to heat build-up and processability.
2. Related Art Statement
In tires for construction vehicles used on mines and stone pits, and large size tires for heavy duty truck and bus frequently running on unpaved road, particularly bad road containing exposed rocks and stones, there is a frequent chance that the tread portion, sidewall portion and the like of these tires are subjected to cut damage by sharp protrusions of the ground stones, and finally the breakage of the tire is caused by the growth of the cut damage or by the corrosion of steel cords as a reinforcement with rain water or the like penetrated through the cut damage.
Therefore, it is desired that the tread portions of the tires for construction vehicle or the large size tires for heavy duty truck and bus are required to have cut resistance and chipping resistance as well as resistance to heat build-up, wear resistance and processability.
In general, it is known that when carbon black is compounded in a large amount as a means for enhancing the cut resistance of the rubber composition, the cut resistance is improved, but the chipping resistance, the resistance to heat build-up or the kneading and extrusion operabilities are considerably poor.
As a means for improving the cut resistance in the starting rubber, Japanese Patent laid open No. 56-82,610 discloses the use of styrene-butadiene rubber having a bound styrene content of 35-55% by weight, and also Japanese Patent Application Publication No. 48-38,615 discloses the compounding of cyclopentadiene resin.
In the above conventional techniques, however, the cut resistance and chipping resistance can be improved, but there are still problems on the wear resistance and processability. Particularly, the heat build-up remarkably increases to undesirably cause the breakage of the tire at high temperature when any restriction is not given to the tire running at high speed under heavy load over a long distance.
Recently, with the advance of performances in the construction vehicle and ore and rubble carrying vehicle, it is demanded to improve not only the resistance to rib tear and chipping resistance but also the wear resistance and resistance to heat build-up in the tires for use in these vehicles.